1.Field of the Invention
The present invention relates to a method of and an apparatus for measuring retardation of a composite layer sample such as a composite sheet prepared by stacking a polarizing film and a phase difference film (retardation film) with each other to be applied to a liquid crystal display panel, for example.
2.Description of the Background Art
As liquid display elements are increasingly employed, requirements for large areas of liquid crystal display panels and enlargement of ranges of visual directions are also increased. Thus, awaited is development of a technique which can readily measure characteristics of a film type polarizing filter (polarizing film) and a phase difference film in steps of producing such films.
In general, the display panel of a liquid crystal display unit has a polarizing filter which is provided on one surface of a liquid crystal cell sealing a liquid crystal material therein as an optical element and a phase difference film, a polarizing filter and a protective film which are successively stacked on another surface (observed side). The phase difference film, which is adapted to compensate polarization characteristics of the liquid crystal cell, consists of a birefringent material. It is necessary to examine the states of a composite sheet, also called an elliptic polarizing plate, which is obtained by stacking and bonding the polarizing filter and the phase difference film with each other, such as the polarization transmission axis direction of the polarizing filter, the optical principal axis direction of the phase difference film and the retardation value thereof. We refer to these characteristic (i.e., retardation value etc.) as "optical characteristic" hereafter.
In the display panel of the liquid crystal display unit, the optical characteristic as viewed from a direction other than that perpendicular to the surface, i.e., the optical characteristic obtained with changing the incident angle of measuring beam, is also an important characteristic. In order to evaluate the angle of visibility, it is necessary to measure the retardation value in the case of changing the angle of incidence of a measuring beam in the state of the composite sheet. At present, however, white light is introduced into the composite sheet to make evaluation by the spectrum of the transmitted light.